The main purpose of metal detectors of the kind described herein is to detect the presence of metal in an article, a bulk material, or generally any object being examined. Such metal detectors are widely used and integrated into production and packaging lines, for example to detect contamination of food by metal particles or components from broken processing machinery during the manufacturing process, which constitutes a major safety issue in the food industry. The generic type of metal detector that this invention relates to and which is known as balanced three-coil system with an encircling coil arrangement can be described as a portal through which the articles and materials under inspection are moving, for example individual packages riding on a horizontal conveyor belt through a vertical portal, or a stream of bulk material in free fall through a vertical duct or funnel passing through a horizontally arranged portal.
The portal is generally configured as a box-shaped metallic enclosure with an entrance aperture and an exit aperture. The operative part of the metal detector is a system of three electrical coils wound on a common hollow carrier or coil former made of a non-metallic material, which is arranged inside the metallic enclosure. The aperture cross-section of the coil former matches the size and shape of the entrance and exit apertures and lines up with them, so that the coil former and the entrance and exit apertures form a tunnel defining a detection zone through which the conveyor belt or other transport means moves the articles or materials under inspection. The aperture cross-section of this detection zone tunnel is generally rectangular or circular, but could also have any other shape.
In state-of-the-art metal detectors of this type, the coils are exactly parallel to each other and, consequently, their parallel planes are orthogonal to their common central axis. The center coil, also called transmitter coil, is connected to a high-frequency oscillator and thus generates a primary alternating electromagnetic field which, in turn, induces a first and a second alternating voltage, respectively, in the two coils on either side of the center coil, which are also called the first and the second receiver coil. The first and second receiver coils are connected in series with each other, but with their windings wired in opposition to each other. In other words, the coil wire runs continuously from a first output terminal through the windings of the first receiver coil, then with the opposite sense of rotary direction through the windings of the second receiver coil to a second output terminal. In addition the first and second receiver coils are located equidistant from the transmitter coil. Therefore, they are in all respects mirror images of each other in relation to the central plane of the transmitter coil, and thus the first and the second alternating voltage induced in them by the primary alternating electromagnetic field will cancel each other. In other words, the mirror symmetry of this state-of-the-art metal detector has the result that the voltage picked up between the first and second output terminals will be zero.
Symmetrical balance coil arrangements can also consist of multiple transmitter coils and/or multiple receiver coils that are arranged in such a way to achieve a so called null balance condition. Therefore the first receiver coil can form one or more entrance-side receiver coils, and the second receiver coil one or more exit-side receiver coils. Likewise the transmitter coil can be designed as one or more transmitter coils.
However, if a piece of metal passes through the coil arrangement, the electromagnetic field is disturbed, giving rise to a dynamic voltage signal across the output terminals of the serially connected receiver coils.
The metallic enclosure surrounding the coil arrangement serves to prevent airborne electrical signals or nearby metallic items and machinery from interfering with the proper functioning of the metal detector. In addition, the metal enclosure adds strength and rigidity to the assembly, which is absolutely essential as even microscopic dislocations of the coils relative to each other and relative to the enclosure can disturb the detection system which is sensitive to signals in the nanovolt range.
An issue of concern in metal detectors of the foregoing description is their sensitivity to stationary and, even more so, to moving metal in areas outside the detection zone and, in particular, even far outside the enclosure of the metal detector. This is due to the fact that the electromagnetic field generated by the transmitter coil extends outside the entrance and exit apertures to a distance as far as two or three times the length of the detection zone. If there are stationary or moving metal parts within this range, for example the support frame or other components of a conveyor, the interaction of the electromagnetic field with the metallic parts in its reach will produce an unwanted output signal of the receiver coils which interferes with the actual detection signals originating from metallic contaminants in the material under inspection traveling through the metal detector. Therefore, unless special design measures are taken, a large space before the entrance aperture and after the exit aperture of the metal detector has to be kept free of all metal. The area that must be kept free of metal is generally called the “metal-free zone” or MFZ.
A more detailed explanation of this requirement of a metal-free zone and a means for reducing or even eliminating the metal-free zone in the type of metal detector described hereinabove are presented in EP 0 536 288 B1, which is hereby incorporated by reference in the present disclosure. One of the possible means for reducing or eliminating the MFZ described in EP 0 536 288 B1 has the form of metallic flanges or collars that may be integral with the rims of the entrance and exit apertures of the enclosure of the metal detector. These flanges or collars act as short-circuit coils in which a current is induced by the alternating electromagnetic field of the transmitter coil. The induced current, in turn, generates a secondary electromagnetic field which can, under certain conditions, nullify the primary field of the transmitter coil beyond a certain distance before the entrance coil and after the exit coil, even to the extent that the primary field outside the apertures of the enclosure is almost totally suppressed and the metal-free zones before the entrance aperture and after the exit aperture are effectively reduced to zero and a so-called “zero metal-free zone” (ZMFZ) can be achieved.
A metal detector using the ZMFZ concept of the foregoing description is especially advantageous for situations where space is restricted, such as with a short conveyor system or when the metal detector is installed in a vertical flow path for example to inspect objects falling inside a chute from a weighing machine to a bag-making machine.
In the last-mentioned case of a vertical arrangement, the chute that guides the falling objects or materials under inspection through the metal detector is in many cases either funnel-shaped or includes funnel-shaped sections. A funnel or generally a conduit with a progressively narrowing cross-section does not match the cylindrical detection zone through a metal detector of the kind described previously. Thus, if the funnel-shaped conduit is matched to the entrance aperture of the enclosure of the metal detector, towards the exit aperture there will be an empty air space of increasing width between the tapered circumference of the funnel and the cylindrical inside wall of the detection zone. This arrangement may be considered sub-optimal in terms of detector sensitivity and space usage. More directly, it points to the need for a metal detector whose entrance and exit apertures and detection zone conform to a tapered, funnel-shaped profile of a channel or chute which guides the movement of the objects or materials through the metal detector. A solution to that need can be provided by an asymmetric configuration of the entire metal detector, wherein not only the exit aperture is smaller than the entrance aperture of the enclosure, but also the coils following each other in sequence, i.e. the entrance-side receiver coil, the transmitter coil, and the exit-side receiver coil, will have to be progressively smaller. At the same time, the advantages of the balanced coil system and of the ZMFZ design concept should preferably be maintained.
It is therefore the object of the present invention to provide a metal detector, for example of the generic type described in the introductory paragraph, with an asymmetrically configured enclosure and detector coil system while maintaining at least the functional properties of a balanced coil system.